Electrode shield



Dec. 15, 1964 H. w. HERBERT ELECTRODE SHIELD Filed Nov. 25. 1960 2 4s. 0 4 Z 3/ r .k a l. 6 m w a v (I? MII%I 3v mm NW m U a \M\ m a a M m E INVENTOR Ham/d H! Heme/'7 fwd/W ATTORNEY United States Patent 3,161,798 ELECTRODE SHIELD Harold W. Herbert, Emporium, Pa., assignor to Sylvama Electric Products Inc., a corporation of Delaware Filed Nov. 25, 1960, Ser. No. 71,731 1 Claim. (Cl. 313-240) This invention generally relates to electron discharge devices and more particularly to internal shield structures used in the manufacture of electron discharge devices such as radio receiving tubes.

Electron discharge devices or tubes are made by a variety of manufacturing techniques. Certain tube types such as those in the miniature or sub-miniature group are fabricated by an electrode stacking technique and have thereby become known as planar or stacked tubes. In this type of tube a plurality of electrodes and their ascsociated spacers are alternately mounted upon support rods. These support rods and spacers are fabricated from ceramic material and are, as their names imply, respectively used to provide the proper lateral support and axial spacing or separation between the operative electrodes of the tube structure. Such tubes are generally very compact and the separations between electrodes are extremely small. Because of this miniature structure, control of current leakage over the spacer surfaces between electrodes is extremely important. This leakage is usually between the grid and cathode or grid and plate electrodes of the tube.

While the spacers described above are normally fabricatedfrom insulating materials such as sintered aluminum oxide, which has high insulating properties, the operation of the tube usually results in a lowering of the interelectrode insulation due to the deposition of conductive material upon the surfaces of the spacers. This conductive material generally consists of particles of active materials such as barium which are sublimated from the cathode as well as metals vaporized during heating of the getter. Additionally, ion bombardment of the cathode surface releases particles of cathode material which are deposited upon adjacent electrode structures.

Therefore, it is an object of this invention to reduce inter-electrode current leakage in stacked tubes and other planar tube constructions due to the deposition of conductive material upon the electrode spacer elements.

It is another object of this invention to lengthen the curent leakage path between alternate electrodes in a tu c.

It is yet another object of this invention to reduce the effect of the migration of active materials such as barium upon a spacer element in an electron tube.

These and other objects of the invention are carried out by provision of a uniquely formed shield element positioned about an insulating spacer which is used intermediate selected tube electrodes that are normally operated at difierent potentials.

For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a side elevation of an electron discharge device duo-triode type mount;

FIG. 2 is an enlarged view of a portion of FIG. 1, showing one specific embodiment of the invention, with some parts sectioned andothers broken away .or omitted in the interest of simplicity and clarity;

FIG. 3 is a perspective view of another embodiment of a spacer shield in partial section; and FIG. 4 is an elevational view of an electrontube utilizing the invention.

Referring to FIG. 1, an electron discharge device mount 8 is shown comprising a pair-of spaced apart support rods 3,161,798 Patented Dec. 15, 1964 10 fabricated flom a ceramic or other insulating material formed to serve as the elements upon which the tube electrodes will be stacked. A first electrode or cathode 12 is separated and insulated from planar grids 14 by spacer elements 16 and spacer shields 18. The anodes 20 for each side of the triode mount are separated from their respective grids 14 by additional insulating spacers 16. All of these electrodes and spacers are longitudinally stacked upon and fixedly attached to the support rods 10 by holding clips 22 clamped on the ends of the rods after assembly.

The thermionic cathode 12 is supported in position by mount 24 which is secured to the cathode base 26 by suitable means such as welding, FIG. 2. A cathode mount of this type is shown in US. Patent No. 2,843,785, assigned to the same assignee as the present invention. Indirectly heated cathode 12, as shown in FIG. 2, has electron emissive material 28 applied to both opposed surfaces. Filament 30, which is used to heat the cathode to its operating temperature, is shown within the interior of the cathode base 26.

The grids 14 comprise a metal base 32 having a centrally disposed aperture 34 and a plurality of spaced lateral wires 36 connected to the side of base 32 disposed adjacent cathode 12 and extended across the aperture. Anodes 20 are fabricated from a suitable metal and have raised central portions 38 formed therein, these central portions being of substantially the same peripheral configuration as grid aperture 34 and in tube axial alignment with aperture 34 and cathode 12.

Spacers 16 are fabricated from ceramic insulating materials such as aluminum oxide, steatite, and the like. They may be circular in shape and have a given thickness selected in accordance with the amount of separation desired between electrodes. The spacers have a centrally located support rod receiving aperture formed therein.

Metal spacer shields 18 are formed as a substantially cup-shaped member having an opening 40 and a base or bottom 42 which is provided with a centrally located sup port rod receiving aperture 44 and a circumferential side wall 46. The inside diameter of the spacer shield wall 46 is larger than the outside diameter of the spacers 16, and the interior height of the shield wall 46 is a given distance less than the thickness of spacer 16 used therewith intermediate mount 24 and grids 14. The end of wall 46 terminates below grid wires 36.

An alternate type of shield 18 is shown in FIGURE 3 as being formed in the same manner as the shield of FIG. 2 but additionally having an insulated coating 48 deposited thereon. Such a structure has increased advantages where the sublimation or leakage problem is more acute which arise from use of the insulation and the longer leakage path of the roughened surface.

Referring again to the specific embodiment of the invention shown in FIGURE 2 as a duo-triode, the lower one of the grids 14 is mounted upon support rods 10 to abut insulated spacers 16. The lower shield 18 is trapped between the grid and the cathode mount support 24, with its opening 40 facing the grid.

On top of the cathode support rests the other shield 18 with a spacer 16 positioned between the base 42 thereof and the second triode grid 14 positioned on the top of this spacer. It can be seen that the thickness of spacer 16 and the thickness of shield base 42 determines the spacing between cathode mount or support 24 and grid of wall 46 of the shield and the outer surface of the spacer 16. Effectiveness of the shield is further dependent upon the distance that the wall extends parallel with the length of the surface of the spacer as measured along the support rod and the position for shading purposes of the top of wall 46 relative to catho'de' 12 and grid 14 Where the grid and spacer 16 abut. The separation between the end of wall 46 and the adjoining electrode or grid 14 should be sufficient so that adequate shielding will occur while arc-over between these electrodes will not occur at any time. The separation between the wall 46 inner surface of shield 18 and the spacer is dependent to some degree upon" the space available in the mount. Generally this spacing may be in the order of .015 inch or lower for sub-miniature tubes. It can, therefore, be seen that the separation between the shield and the spacer is extremely small yet very efiectiv'e.

Since material which is sputtered or Sublimated from the cathode material coating 28 is generally deposited upon the cooler surfaces adjoining the cathode structure, it can be seen that by interposing shield 13 between the cathode mount 24 and spacer 16, the spacer is protected fromhaving conductive paths formed thereon. In addition, even after many hours of tube operation, there is little likelihood of current leakage between the cathode and grid due to the circuitous path around wall 46 and over the length of spacer 16';

While the embodiment of the invention described herein deals with shields positioned between the cathode 12 and grids 14, it will be understood that shields may also be used between any pair of adjoining tube electrodes which are normally operated at dilferent potentials such as grids 14' and anodes 20, thereby further reducing the possibility of having any undesirable current leakage paths formed due to the sublimation or deposition of conductive materials in the tube.

It will be apparent to those skilled in the art that various changes and modifications may be made in the structure described herein without departing from the scope of the invention as defined by the appended claim.

What is claimed is:

An evacuated electron discharge device comprising a thermionic cathode employing materials which sublime during heated cathode operation, a metal support for the cathode, at least one electrode normally operated at a potential different than said cathode and spaced therefrom, an insulating spacer of a given thickness abutting said electrode, and a metallic shield having an insulating coating thereon, said' coating providing a surface that is relatively rougher than the surface of the metal and said shield having a base of a given thickness abutting said insulating spacer on one surface and said support on the opposed surface whereby the spacer and the shield base given thicknesses additively determine the cathode to electrode spacing, said shield having a wall upstanding from the base thereof positioned a given offset distance from said spacer disposed intermediate said cathode and spacer, the end of said wall terminating a given distance away from said electrode, said given distance being greater than the arcing distance in said evacuateddevice between the electrode and said wall and less than the distance required to shield substantially all of said spacer from deposition thereon of the subliming cathode mate'- rial.

References Cited in the file of this patent UNITED STATES PATENTS 2,508,992 Coolidge et a1 May 23, 1950 2,899,590 Sorge't a1. Aug. 11', 1959 3,070,724 Herbert et a1. Dec. 25, I962 

